Epitaxial deposition process



April 25, 1967 B. LOMBOS ETAL 3,316,121

E PITAXIAL DEPOSITION PROCESS Filed Oct. 7:, 1965 3 Sheets-Sheet 1 POWERSOURCE BELA LOMBOS and THOMAS RAPHAEL SOWGYI y W, W M Attorneys April25, 6 B. LOMBOS ETAL 3,316,121

EPITAXIAL DEPOS ITION PROCESS Filed Oct. 5-2, 1963 3 Sheets-Sheet P;

W W/ M 2 M; A

V DIRECTION OF GAS FLOW RUN NO.| STANDARD DEVIATION =04,

RUN NOZ STANDARD DEVIATION =O-4JL BROWN ISH DEPOSIT BELA LOMBOS andTHOMAS RAPHAEL SOMOGYI By W Dra W eys April 25, 1967 B. LOMBOS ETAL 3,

EPITAXIAL DEPOSITION PROCESS Filed Oct. 2 1963 3 Sheets-Sheet 8 I700 f 9cd b a TOK RESISTANCE RF HEATING HEATING A E 60 kCGI/IIIOIQ G d AE= 37kcoI/mole b e I400 AE= 24 kcoI/mole c f I I I I I I 0 2 4 6 8 IO [2 I4DISTANCE IN CM BELA LOMBOS and THOMAS RAPHAEL SOMOGYI United StatesPatent Ofiice 3,3121 Patented Apr. 25, 1967 This invention relates to amethod and apparatus for the epitaxial growth of crystals and moreparticularly concerns the epitaxial deposition of silicon from a vapouronto a silicon substrate.

As is known, the epitaxial deposition of a substance onto a singlecrystal substrate of the same substance involves the oriented overgrowthon the substrate so that its surface provides through its latticestructure, preferred orientation for the deposited material. Epitaxialgrowth of semi-conductor crystals is of great commercial interest andutility in the electronics field.

In the past, difiiculty has been encountered in epitaxial depositionprocesses since the growth conditions of each specimen have to becarefully watched and controlled if uniform thicknesses of deposition ona single specimen and from one to the next are to be obtained. Inparticular, for example, when a number of specimens are prepared in asingle reaction tube, or vessel in a moving vapour stream by thereduction of silicon tetrachloride by hydrogen, the depletion of thesilicon tetrachloride from the gaseous phase leads to excessivedeposition on the substrate specimens first encountered and insufficientdeposition on those encountered later by the gas phase. To some extent,this trouble has been mitigated in trying to keep the concentration ofreactants and products constant by arranging for the inlet of gasmixture at different locations in the vessel. It is not, however,entirely satisfactory and is extremely complex to achieve properly.

It is an object of the present invention to meet the difficulties of theprior art, and there is provided a method of epitaxial deposition of asolid from a gaseous phase comprising the steps of, controlling theequilibrium constant of the reaction so that when depletion of thereactants tends to alter the reaction rate, the equilibrium constant isadjusted to maintain a constant rate of deposition.

Now, Miller et al. report in the Journal of the Electrochemical Society,109, p. 643 (1962), that the free energy change for the reaction:

(over the range 298-2000" K.)

Thus, we may deduce that the equilibrium constant K of the reaction willbe given by:

It will be seen therefore that log K and hence K increases withincreasing temperature within the practical range for single crystalgrowth. Now as mentioned above, in a uniform temperature reaction tubethe yield of the reaction decreases in the direction of the gas flowbecause of changes in the concentrations of reactants and products. Whenhydrogen is in a very large excess over the incoming SiCl itsconcentrationmay be regarded as substantially constant. Qualitatively,then, there occurs a gradual drop in the mole fraction of SiCl,accompanied by a rise in the mole fraction of HCl as the gaseous mixturetravels along the tube, resulting in a decreasing deposition rate. -Wefind that if the equilibrium of the reaction can be altered by adjustingthe temperature profile of the furnace according to the desired changein equilibrium the deposition rate along the reaction tube can be keptsubstantially constant. Since the equilibrium constant increases withthe temperature, the usable portion of the furnace is determined by themelting point of silicon on the one hand and the lowest temperature forsingle crystal deposition of satisfactory qualtity on the other hand(approx. 1400-1700 K. for silicon). We find that the flow rate has aneffect on this applicable length too.

After a discussion of the theory, a description of an apparatus forcarrying out the invention will be made and reference will be had to thedrawings in which,

FIGURE 1 shows a partly schematic arrangement of an apparatusconstructed in accordance with the teaching of the invention,

FIGURE 2 shows a view in detail of the furnace in FIGURE 1.

FIGURE 3 is a plan view of epitaxially deposited samples showingnumerically the thickness of deposit,

FIGURE 4 is a series of graphs of distance alongthe reaction tubeagainst temperature for various values of AB.

The evaluation of the temperature gradient for a given flow rate using atubular reaction chamber can be made in the following way:

The deposition rate (say) can be expressed in the form where or is anexperimental parameter involving vapor saturator temperature, geometryof apparatus etc. (but not x or T), x is the distance along thedirection of gas how, and T is the absolute temperature.

Therefore,

tip-20 da aw+ dT where or, are variables, independent of each other andof x and T.

The condition for uniform deposition is:

2 0 ln f H J;

@T at Finally, integrating Equation 3:

T a in y o- 111 f :60

where R is the gas constant and AE the activation energy. When thisequation is integrated without fixing upper and lower limits, andtherefore introducing an arbitary constant of integration, we have(assuming AE contant) Now, if all parameters except temperature arefixed, Equation 1 will read:

i.e. g may be substituted for [L in Arrhenius equation. Hence, theintegrated form will become:

AE=60 kcal./mole AE=37 kcal./mole AE=24 kcal./mole respectively +const.

We also determined the function in f(x) experimentally by employing botha resistance heated and an RF. heated furnace using a constanttemperature range and obtained two different results.

To obtain the function ln f(x) slices of silicon were placed in auniform temperature section of a reaction tube at 1215 C. and thedeposition rate as a function of distance was measured. A similarexperiment was conducted in an R.-F. heated reaction chamber resultingin a different set of values for In f(x). The two sets were fitted tocurves assuming a linear relationship between In ,u. and x. Thefunctions representing those curves were found to be:

. ln =6t5307 1O -2.4804 l x (Standard Deviation=3.9l X

for the resistance heated tube, and

In ,(L=1.5048-5.2600' 10" x (Standard Deviation: 3 .5 42 10- for the RF.heated tube.

These results combined with the above two values for the activationenergy give a total of six distinct curves (a to f) for the temperatureprofile (see FIG. 4). In practice, the most uniform deposition with ourresistance heated furnace was achieved with a temperature gradientclosest to curve a of FIG. 4.

An apparatus for obtaining substantially uniform deposition of siliconwill now be described having reference to FIGURE 1. A furnace 1 heatedby source 10 and in a manner which will be described later includes areaction tube 2 within which are placed specimens 3 of single crystalsilicon upon which epitaxial deposition is to take place. Hydrogen isintroduced into this tube through a line 4, whence it passes to anelectrically operated routing and shutoff device shown diagrammaticallyas a valve 5. The hydrogen then passes into a line 6 whence it passesinto a container 7 surrounded by liquid nitrogen 8. This nitrogen servesto cool the hydrogen and any gases boiling at a temperature higher thannitrogen are trapped by the molecular sieves 9 within the container 7.The cleaned hydrogen leaves vessel 7 through a conduit and is then:allowed either to pass into pipe 16 or pipe 17 depending upon thesetting of valves 18 or 19 respectively. The ;gas passing through eitherof these tubes also encounters a flow meter 20 or 21 respectively,containing a ball 22 or 23 which is supported by the gas and is thusfree to indicate rate of flow of gas through the tube concerned. Gaspassing through tube 17 enters tube 25. From this the gas may pass intoa silicon tetrachloride saturator by valve 27 or through a bypass line28. The saturator 26 comprises a liquid containing vessel Within whichis placed a liquid that will not freeze at the temperature at which thesilicon tetrachloride is to be held for evaporation by the hydrogen. Amixture of ethylene glycol and water is suitable. The mixture is cooledby refrigeration coils 40. Within the liquid 36 is suspended a filtervessel 37, evaporator 38 and further filter vessel 39. These vessels areinterconnected consecutively, the silicon tetrachloride containingvessel being so arranged that the hydrogen is introduced into thetetrachloride through a perforated disc and allowed to bubble up throughit thereby mixing with and carrying off the vapour. After the mixtureleaves the filter 3 it passes through the two way valve 29 and into thereaction tube 2.

A two way valve 5 is also arranged so that hydrogen may be excluded andhelium introduced through a pipe 45. The helium may thus pass throughthe saturator and/or the bypass, depending upon the setting of valves 18and 19.

Gas leaving the reaction tube 2 in a commercial establishment would bereclaimed, but in a laboratory setup it may be burned to avoidcontaminating the atmosphere with explosive hydrogen as shown by the gasflame 46. This gas contains a certain amount of hydrogen chloride whichshould therefore be disposed of by venting into a fume cupboard in theconventional manner.

Turning now to FIGURE 2, in the furnace 1, gas enters tube 2 as shown bythe arrow 50 and passes over the specimens 3 spaced out in a lengthwisedirection along the tube on supports 51. If the tube 2 is ofconsiderable width as shown here, more than one specimen may be placedin the cross section. Alternatively there may be a single holder 51extending along the tube on which the silicon specimens 3 are placed.The furnace is heated by a coil 52 shown as a resistance heating devicewhich may be embedded in firebrick 55. By providing a greater number ofcoils at the end of the tube at which the gas exits than at the entranceto the tube, an increasing temperature profile along the tube can bedeveloped. The profile may be altered by changing the spacing of thecoils, and the temperature at any cross section is dependent uponcurrent through 52. Surrounding the coil is a tapered tube of insulatingmaterial 53 which again may be varied in thickness as required so as toalter the quantity of heat escaping and to cause more or less heat to betransferred from the heating coil 52 to tube 2. The temperature alongthe length of the tube may be determined by thermo-couples such as 54inserted into it.

In one experimental apparatus, the furnace 1 was of firebrick with aKanthal Al heating element embedded in it. The reaction tube was offused quartz about centimeters long and 40 millimeters in diameter.

An alternative heating method is by electro-magnetic induction. The coil52 may carry a high frequency current and since it will develop a moreintense field where there are a larger number of loops a desiredtemperature profile within the tube can again be developed, by applyinga suitable material for coupling (such as graphite) to the surface ofthe tube. The temperature may be measured by inspection through aportion of the tube with a radiation pyrometer.

In the prototype apparatus of FIGURES 1 and 2, operation was begun by,arranging on a quartz holder a single row of slices of siliconhorizontally over a distance of 8 cm. along the length of the tubeswitching on the heating element 52. The apparatus was first purged bypassing helium for fifteen minutes through both lines 16 and 17. Thesaturator 38 was bypassed by the helium by allowing it to fiow throughline 23. After that the deposition of silicon was allowed to take placeby altering the position of valve 5 to allow hydrogen to enter throughpipe 4 and to pass through line 16 and saturator 38. A flow rate ofabout 0.2 litres of hydrogen per minute was arranged through thesaturator whose temperature was kept at 18 C. by means of the bathliquid 36, and 6 to 8 litres of hydrogen per minute was passed throughthe bypass line 16.

The highest temperature that could be reached with the Kanthal Al wirefurnace was just over 1300 C. and the slices were in the range oftemperature between 1200- 1300 C. Two runs were made under the followingconditions:

Deposition thickness was measured by an infra-red interference fringemethod disclosed by W. G. Spitzer and M. Tanenbaum in the Journal ofApplied Physics 32, page 744 (1961) and the results for two depositionruns are shown in FIGURE 3. The figures represent thickness of depositin microns and it can be seen that the Standard Deviation of thedeposited layer thickness over all the four slices of any particular rundid not exceed 0.4 .t.

The reason for finding curve a of FIG. 4 closest to the temperaturegradient producing the most uniform deposition for our resistance heatedfurnace can be explained in terms of diffusion control of the reaction.The experimental values of AE for our R.F. heated furnace and found byTheuerer were determined using slices of silicon with their planesperpendicular to the direction of the flow whereas in our resistancefurnace they were parallel to the flow. We find the effect of the flowrate on the structure of the adsorption layer on the silicon is moresignificant for perpendicularly oriented slices than parallel ones.

In our resistance heated furnace a predeposition of silicon was formedon the hot wall of the reaction chamber, altering the concentrations ofthe gases arriving at the slices. The value of the activation energydetermined experimentally in a constant temperature region of theresistance furnace under the above mentioned conditions was 60 kcaL/moleas stated before and using this value the desired temperature gradientalong the furnace, producing the most uniform deposition was found(curve a, FIGURE 4). With the RF. furnace and having the slices withtheir planes perpendicular to the gas flow the value of 24 kcal./molewould give the temperature gradient with the most uniform deposition. Inother instances where a furnace with other characteristics might beused, the value of AE found for that furnace for a given orientation ofthe silicon specimens in the gas stream and flow velocity would yieldthe temperature profile for most uniform deposition.

We claim:

1. The method of depositing a solid from a gas phase epitaxially onto asolid phase wherein a gas thermally convertible to solid is passed overand along a crystalline surface of a solid upon which epitaxialdeposition is to be made, the rate of deposition of solid from said gasbeing temperature dependent, said surface extending in the direction offlow of said gas, said deposition reducing the concentration of activeelements in said gas whereby altering the rate of deposition of solid asthe gas progresses along the surface, which comprises the step of,varying the temperature of said surface, in the direction of gas flow,for altering the equilibrium constant for the gas/ solid reaction in adirection to increase the rate of deposition, for a given concentrationof active elements in the gas, as the concentration of said elements insaid gas is reduced.

2. The method as defined in claim 1, the temperature of said surfacebeing varied in the direction of said gas flow for maintaining asubstantially constant rate of deposition of solid from said gas ontosaid surface.

3. The method of uniformly depositing a solid from a gas phaseepitaxially onto a solid phase wherein a gas thermally convertible tosolid is passed over and along a crystalline surface of a solid uponwhich deposition is to be made, said surface extending in the directionof flow of said gas, said deposition reducing the concentration ofactive elements in said gas, whereby altering the rate of deposition ofsolid as said gas progresses along the surface, which comprises thesteps of, estabilshing a chosen uniform temperature over aid surface,passing said gas over said surface for a predetermined time interval,measuring the thickness of deposit for chosen distances along saidsurface in the direction of gas flow, and modifying the temperature ofsaid surface progressively in the direction of gas flow to achieve auniform thickness of deposit over the entire surface.

References Cited by the Examiner UNITED STATES PATENTS 2,880,117 3/1959Hanlet 117-106 2,877,138 3/1959 Vodonik 118-491 X 3,031,338 4/1962Bourdeau 117-107.2 X 3,168,422 2/1965 Allegretti et a1. 23223.5 X3,201,101 8/1965 Jacques 118-49.1 X

ALFRED L. LEAVITT, Primary Examiner. A. GOLIAN, Assistant Examiner.

1. THE METHOD OF DEPOSITING A SOLID FROM A GAS PHASE EPITAXIALLY ONTO ASOLID PHASE WHEREIN A GAS THERMALLY CONVERTIBLE TO SOLID IS PASSED OVERAND ALONG A CRYSTALLINE SURFACE OF A SOLID UPON WHICH EPITAXIALDEPOSITION TO TO BE MADE, THE RATE OF DEPOSITION OF SOLID FROM SAID GASBEING TEMPERATURE DEPENDENT, SAID SURFACE EXTENDING IN THE DIRECTION OFFLOW OF SAID GAS, SAID DEPOSITION REDUCTING THE CONCENTRATION OF ACTIVEELEMENTS IN SAID GAS WHEREBY ALTERING THE RATE OF DEPOSITION OF SOLID ASTHE GAS PROGRESSES ALONG THE SURFACE, WHICH COMPRISES THE STEP OF,VARYING THE TEMPERATURE OF SAID SURFACE, IN THE DIRECTION OF GAS FLOW,FOR ALTERING THE EQUILIBRIUM CONSTANT FOR THE GAS/ SOLID REACTION IN ADIRECTION TO INCREASE THE RATE OF DEPOSITION, FOR A GIVEN CONCENTRATIONOF ACTIVE ELEMENTS IN THE GAS, AS THE CONCENTRATION OF SAID ELEMENTS INSAID GAS IS REDUCED.